Physics

Light

-Mirror Images, Reflection, Refraction, Total internal Reflection…

Laws of Reflection

-          The incident ray, reflected ray and normal to the surface all lies on the same plane.

-          The angle of incidence is equal to the angle of reflection

Regular Reflection

-          Light is reflected in one direction only.

-          Occurs on smooth surfaces such as mirrors or still clear water surfaces.

Irregular / Diffused Reflection

-          Light is reflected through a range of different angles.

-          Occurs on rough surfaces such as unpolished walls or cloth.

Effects of Reflection

Effect	Explanation
Swimming pool always appears to be shallower than it really is.	Light from the bottom of the pool is refracted away from the normal, traveling into air, as the speed of light increases when traveling from dense (water) to less dense (air) medium.
A stick partially submerged in water appears to be bent.	Light from the submerged part of the stick is bent away from the normal, towards the eye. This refraction makes the part of the stick under water appear nearer and hence, look bent.
Writing below a piece of thick glass block appears nearer.	Rays of light are refracted away from the normal as they leave the glass. When they are received by the eye, they appear to come from a point nearer.

 

Mirror Images

Characteristics of Images formed in a plane mirror

-          Same size as object.

-          Same distance behind the mirror as the object is in front.

-          Laterally inverted.

-          Virtual

-          Upright

Laws of Refraction

-          The incident ray, normal and refracted ray all lies on the same plane.

-          For two particular media, the ratio of the sine of angle of incidence to the sine of angle of reflection is a constant. 

Refractive Index

-   is also known as the refractive index of a medium where the incident ray is in the air and refracted ray is in the medium.

When light travels from a less dense medium to a denser medium, it bends towards the normal.

Angle of incidence is larger than the angle of refraction.

When light travels from a denser to less dense medium, light bends away from the normal.

Angle of incidence is smaller than the angle of refraction.

Total internal Reflection

Critical Angle – The angle of incidence in the optically denser medium for which the angle of refraction in the less dense medium is 90°.

Total internal Reflection occurs when

-          Ray of light travels from a denser to less dense medium.

-          Angle of incidence in denser medium is greater than critical angle.

Relationship between critical angle and refractive index of denser medium:

Sin c =

Applications of total internal reflection

Applications	Description
Periscopes	Periscopes are constructed using two right-angled prisms. The light rays hit the inside surface of the prisms at angles greater than the critical angle, making them internally reflected.
Binoculars	Binoculars make use of prisms to reduce the length of the instrument and to produce an erect image. The light rays are bent through 180° by each prism in contrast to the periscope where light rays are only bent through 90° by each prism.
Optical Fibres	An optical fibre is made of a core of high reflective index glass of plastic. It is usually coated with a glass of lower refractive index. A light ray introduced into the optical fibre will be internally reflected at the surfaces. Even though the optical fibre may be bent, the light will still be internally reflected. Optical fibres are used in telecommunication industries as they can carry much more information than copper wires, and are lighter and cheaper to manufacture. Lenses Focal Length Optical Centre – The point midway, between the lens’s surfaces on its principal axis. Principal axis –        The line passing symmetrically, through the optical center of the lens. –          Focus / Focal point –        The point, which all rays close to and incident parallel to the principal axis converge to after refraction by the lens. (Rays parallel to principal axis, converged after refraction, meets at the focal point, on the principal axis.) Focal Length –        The distance between the optical center and the principal axis. Finding Focal Length: The lens apparatus are arranged as shown above. A distant object is chosen so that light rays from it may be regarded as parallel to the principal axis. The position of the white screen is adjusted until a clear / sharp image ( inverted and diminished ) is formed on it. The distance between the optical center of the lens and the image is measured using a ruler. This distance is the focal length. Drawing and uses of lenses Position of Object Position of Image Real or virtual Inverted or Erect Magnified or Diminished Uses At Infinity At F Real Inverted Diminished Camera, Eye Outside 2F (Beyond 2F) Between F and 2F Real Inverted Diminished Camera, Eye At 2F At 2F Real Inverted Same Size Photocopier Between 2F and F Beyond 2F Real Inverted Magnified Projector At F At Infinity Real Inverted Magnified Spotlight Between F and lens Behind lens Virtual Erect Magnified Magnifying Glass *Note: Eye is not a equipment. Measurement S.I. Units No. Basic Physical Quantity Symbol for Quantity Name of S.I. Unit Symbol for S.I. Unit 1. Length l Metre m 2. Mass m Kilogram kg 3. Time t Second s Area cm² Volume m³ Instruments used for Measurement No. Length to be measured Suitable instrument Accuracy of Measurement 1. Several metres Measuring tape 0.1 cm 2. Several cm to 1 cm Metre or half-metre rule 0.1 cm 3. Between 1 cm and 10 cm Vernier Calipers 0.01 cm 4. Less than 2 cm Micrometer Screw Gauge 0.01       cm ( 0.01 mm) Reading a Vernier Caliper Reading a Micrometer Screw Gauge Refer to TB.   Experiments on Density To find the density of a regularly shaped object Apparatus:       -    Vernier Calipers -         Ruler -         Electronic Balance Mass: The mass is formed by using an electronic balance. The appropriate measurement of the object is taken. The volume is calculated using the formulae: Volume: Cuboid : Length, Breadth and Height are measured using the vernier calipers. Volume= length x breadth x height Cylinder : The diameter and the length are measured by using a pair of vernier calipers or ruler Sphere : The diameter is measured using a pair of vernier calipers Volume= 4/3p(d/2) ² Density: The density is calculated using the formulae, Density = Mass/Volume Precautions: Readings from the vernier calipers and ruler are without parallax error. To find density of an irregular object that sinks Apparatus: -Electronic Balance -Measuring Cylinder -Displacement Can -Thread -Object Mass: The mass is found by using an electronic balance. Volume: Method 1; for small objects: A measuring cylinder is filled with water up to a volume V1. The solid is lowered gently into the measuring cylinder until the solid is completely immersed in water. The new volume V2 is read. Volume of object = V2-V1 Method 2; for big objects: Displacement method. A displacement can is filled with water to the brim/ until it is overflowing. When it has stopped overflowing, the solid is lowered completely into the can with a measuring cylinder just at the spout of the can to catch the displaced water. Volume of Object = Volume of displaced water in the measuring cylinder. Density: Measured using formulae Density = Mass/Volume. Precautions: -The solid is lowered gently without splashing of water from the measuring cylinder or displacement can -The cylinder should be placed on a flat surface while reading the volume . -The eye should be at the same level as the base of the meniscus when reading the volume. To find density of an irregular object that floats Apparatus: -Electronic Balance -Measuring Cylinder -Thread -Object -Sinker Mass: The mass is found using electronic balance Volume: A sinker is immersed into a measuring cylinder filled with water; this volume V3 of the sinker is read. The solid tied to the sinker is lowered gently into the water until the solid and sinker are immersed in the water completely. The new volume V4 is read. Volume = V4-V3 Density: Calculated using formulae Density= Mass/Volume Precautions: -The solid is lowered gently without splashing of water from the measuring cylinder. -The measuring cylinder should be placed on a flat surface while reading volume. -The eye should be same level as the base of the meniscus while reading the volume of water. To find density of liquid Apparatus: -Electronic Balance -Beaker -Liquid Mass: The mass of a clean, dry beaker, m1, is measured using an electronic balance. The liquid is poured into the beaker. The mass of the beaker, with the liquid m2, is measured by the electronic balance. Mass of liquid = m2-m1 Volume: The volume of the liquid is measured from the beaker. Density: Calculated by formulae Density=Mass/Volume. Precaution: -The beaker should be dry at the beginning -When reading the volume of the liquid, make sure that the eye is level with the base of the meniscus of the liquid. -The beaker should be on a flat surface when reading the volume. Distance-Time graph Gradient is numerically equal to the velocity. Distance moved can be read from y-axis. Velocity-Time graph Gradient is numerically equal to the acceleration. Distance moved is area under the graph. Acceleration -         The change in velocity per unit time. -         Symbol = a. Acceleration =                                                                                                                                   . Scalar Quantity UNFINISHED—UPDATE!!!